Field of the Invention
The present invention relates to a method and an apparatus for polishing a substrate having a silicon layer forming a surface thereof, and more particularly to a polishing method and a polishing apparatus capable of detecting a polishing end point of the substrate based on optical information contained in reflected light from the substrate.
Description of the Related Art
Semiconductor devices are fabricated through several processes including a process of polishing a dielectric film, e.g., SiO2, and a process of polishing a metal film, e.g., copper or tungsten. A fabrication process of backside illumination CMOS sensor includes a process of polishing a silicon layer (silicon wafer), in addition to the polishing processes of the dielectric film and the metal film. The backside illumination CMOS sensor is an image sensor using BSI (backside illumination) technique and has a light-receiving surface constituted by the silicon layer. A fabrication process of through-silicon via (TSV) also includes a process of polishing a silicon layer. The through-silicon via is an electrode constructed by metal, such as copper, formed in a hole passing through the silicon layer.
Polishing of the silicon layer is terminated when its thickness has reached a predetermined target value. CMP (Chemical Mechanical Polishing) apparatus is used for polishing the silicon layer. FIG. 1 is a schematic view of the CMP apparatus. The CMP apparatus includes a polishing table 101 with a polishing pad 100 attached to an upper surface thereof, a top ring 110 for holding a wafer W, a slurry supply unit 115 for supplying a polishing liquid (slurry) onto the polishing pad 100, and a film-thickness measuring device 120 for measuring a film thickness of the wafer W. The film-thickness measuring device 120 is embedded in the polishing table 101.
The top ring 110 and the polishing table 101 rotate as indicated by arrows. In this state, the top ring 110 presses the wafer W against the polishing pad 110, while the slurry supply unit 115 supplies the polishing liquid onto the polishing pad 110. The wafer W is polished by sliding contact with the polishing pad 110 in the presence of the polishing liquid. During polishing of the wafer W, the film-thickness measuring device 120 rotates together with the polishing table 101 and measures the film thickness while sweeping a surface of the wafer W. Polishing of the wafer W is terminated when the film thickness has reached the predetermined target value.
An optical film-thickness measuring device is one example of the film-thickness measuring device 120 used in the above-described CMP apparatus. This optical film-thickness measuring device is designed to direct light to the surface of the wafer and analyze reflected light from the wafer to determine a thickness of a film formed on the surface of the wafer. Silicon (Si) has a larger refractive index than that of dielectric material, such as SiO2, and hardly allows visible light to pass therethrough. Thus, instead of the visible light, infrared ray, which has a good permeability, is used in measuring of the thickness of the silicon layer.
The film-thickness measuring device using the infrared ray can measure the thickness of the silicon layer. However, the film-thickness measuring device incorporated in the CMP apparatus may fail to measure the thickness if there is a variation in the thickness of the silicon layer because it measures the thickness of the silicon layer while the device itself is moving as shown in FIG. 1. In particular, the silicon layers that have been polished in the above-mentioned BSI fabrication process and the TSV fabrication process have a polished surface with low flatness, which is likely to cause the failure of measurement. One possible solution to such measurement failure is to shorten a measuring time per one measuring operation. However, in this case, a quantity of the reflected infrared ray is reduced and as a result S/N ratio is lowered. This makes it difficult to achieve accurate measurement. Moreover, since the silicon layer is scraped away every time the polishing table makes one revolution, it is impossible to measure the thickness at the same position under the same conditions.